1. Field of the Invention
The present invention relates to a secondary air supplying structure of an internal combustion engine, which supplies part of intake air flowing through an intake system of the internal combustion engine or outside air, i.e., so-called secondary air, to an exhaust system of the engine.
2. Description of the Background Art
An example of vehicle-mounted internal combustion engines is equipped with a secondary air supplying apparatus for supplying secondary air to an exhaust system of the engine in an attempt to improve exhaust emissions. With the secondary air supplying apparatus, for example in a period in which the engine was cold-started and a catalyst arranged at some midpoint in the exhaust system is not activated yet, the secondary air is supplied to the exhaust system of the engine through a secondary air supplying path. When the exhaust system is supplied with the secondary air, unburned gases contained in the exhaust gases within the exhaust system are burned, and hydrocarbon (HC) and/or carbon monoxide (CO) in the exhaust gases are oxidized. Such oxidation of HC and/or CO in the exhaust gases reduces the remaining amount of HC and/or CO, whereby their emission to the outside in the period in which the catalyst is not activated is suppressed and the exhaust emissions are improved. Additionally, the combustion of the unburned gases warms up the catalyst quickly, whereby the catalyst enters the active state at an early stage after the cold-start of the internal combustion engine. Such a secondary air supplying structure of an internal combustion engine is disclosed in Japanese Utility Model Laying-Open No. 05-010735 (hereinafter referred to as Reference 1) and Japanese Patent Laying-Open No. 2003-035136 (hereinafter referred to as Reference 2).
Reference 1 discloses a structure with a secondary air supplying pipe arranged along the longitudinal direction of a cylinder head and a secondary air distributing pipe for distributing the secondary air from the secondary air supplying pipe to each cylinder exhaust system. According to Reference 2, a delivery passage constituting the secondary air supplying pipe is integrally formed with a flange of an exhaust manifold. Now, while it may be possible to integrally join the secondary air supplying pipe disclosed in Reference 1 to the flange of the exhaust manifold disclosed in Reference 2 and to form the secondary air distributing pipe at the flange of the exhaust manifold, it is not easy to integrally join the secondary air supplying pipe to the flange of the exhaust manifold.